In recent years, an alkaline storage battery has drawn attention as a power source for a portable instrument or devices or also as a power source for an electric vehicle, a hybrid electric vehicle, or the like. As such an alkaline storage battery, various types have been proposed. Among them, a nickel-metal hydride secondary battery comprising: a positive electrode made of an active material primarily containing nickel hydroxide; a negative electrode containing a hydrogen absorbing alloy as a main component; and an alkaline electrolyte containing potassium hydroxide or the like has rapidly become widespread as a secondary battery having a high energy density and excellent reliability.
The positive electrodes of nickel-metal hydride secondary batteries are roughly divided into two types depending on the difference between production methods therefor, which are a sintered nickel electrode and a paste (non-sintered) nickel electrode. Of the two types, the sintered nickel electrode is produced by precipitating nickel hydroxide in extremely fine pores in a porous sintered substrate obtained by sintering nickel fine powder onto the both sides of a punched steel plate (punching metal) by a solution impregnation method or the like. On the other hand, the paste nickel electrode is produced by filling an active material containing nickel hydroxide directly into fine pores in a high-porosity substrate using a foamed nickel porous body (formed nickel substrate) (see, e.g., Patent Document 1).    Patent Document 1: Jpn. unexamined patent publication No. 62 (1987)-15769    Patent Document 2: Jpn. patent No. 3363670    Patent Document 3: Jpn. patent No. 3234492    Patent Document 4: Jpn. unexamined patent publication No. 2001-357844
Since the paste nickel electrode is high in the filling density of nickel hydroxide and easy to be increased in energy density, it has currently become the main stream of a positive electrode for a nickel-metal hydride storage battery. In recent years, attempts have been made to improve the active-material utilization ratio, high-rate discharge characteristic, and output characteristic of the paste nickel electrode by improving active material powder to be filled therein, a material to be added thereto, and the like (see, e.g., Patent Documents 2, 3, and 4).
In Patent Document 2, a thermal treatment is performed in the coexistence of oxygen and an alkali so that a coating layer made of a cobalt compound having a valence larger than 2 and a disturbed crystallinity is formed on the surface of each of nickel hydroxide particles containing one or more of zinc, cadmium, magnesium, and calcium. Since a cobalt compound having a valence larger than 2 has an extremely high conductivity, it is reported that the active-material utilization ratio is significantly improved. In addition, it is also reported that, by causing one or more of zinc, cadmium, magnesium, and calcium to be contained in a solid solution state in each of the nickel hydroxide particles, a reduction in capacity during over-discharging can be suppressed, while a high active-material utilization ratio is retained.
In Patent Document 3, a conductive layer made of a sodium-containing cobalt compound of which the content of sodium is adjusted to 0.1 to 10 wt % is formed on the surface of each of nickel hydroxide particles or grains containing nickel as a main component. It is reported that, with sodium being captured in the crystal of a cobalt compound, the compound having a high conductivity is provided and the active-material utilization ratio is thereby significantly increased.
Patent Document 4 has proved that, by using a solid solution of nickel hydroxide containing a small amount of magnesium, an excellent high-rate discharge characteristic and an excellent output characteristic are shown. By further using the solid solution of nickel hydroxide containing a small amount of magnesium for a positive electrode active material, the generation of γ-NiOOH can be suppressed so that the cycle lifetime of a battery is also improved.